1. Field of the Invention
This disclosure relates to a sealed, blind, internally threaded fastener assembly that is cold expanded and at least partially collapsible to form a secure fit within a work piece.
2. Description of the Related Art
The ability to blindly install a fastener into structural work members is well known in the art, particularly in the aerospace industry. As used herein, a blind fastener comprises the nut or receiving portion of a bolt/nut fastener group. Blind fasteners are often used for access panels and other structure where it is difficult or nearly impossible to get a tool onto the back or “blind” side of the panel to hold a nut. Thus the blind fastener, once installed, performs like a nut to receive an attachment bolt so that other structure or accessory devices can be attached to the work piece or access panel, for example. A blind fastener, sometimes referred to as a blind rivet, is typically a cylindrically shaped device with an internally threaded passage.
The blind fastener or rivet is installed into an opening in a work piece from an access side of the work piece. Blind fasteners are typically provided with an integrally formed closed end and internal threads machined within a partial opening in the fastener body. The blind fastener can have a first flange or a head on one end for seating the blind fastener against the access side of the work piece and for helping to axially restrain the blind fastener with respect to the work piece. The opposite end of the blind fastener, which is inserted through the opening in the work piece, is configured with internal threads. The internal threads permit the blind fastener to be attached to an installation tool during installation of the blind fastener and further permits an attachment or accessory bolt to be threaded into the blind fastener when, for example, an accessory device is mounted to the work piece.
A typical installation tool used to install a blind fastener has a threaded rod coupled to the tool. The tool is operated to pull on the rod, thus causing a portion of the blind fastener located on the blind or inaccessible side of the work piece to collapse and form a second flange. The second flange, formed on the blind side of the work piece works in conjunction with the head of the blind fastener to axially secure the blind fastener within the work piece (e.g., axial with respect to the longitudinal axis of the blind fastener). Blind fasteners or rivets of the type just described are currently provided by Bollhoff Rivnut company, with headquarters in Bielefeld, Germany under their RIVNUT® brand of fasteners.
To enhance the fatigue life of the surrounding work piece, and/or to rotationally secure the blind fastener with respect to the work piece, at least a portion of the blind fastener can be radially and plastically expanded into the work piece during installation. Methods and tools that can be used to install blind fasteners or rivets are described in detail in U.S. Pat. No. 6,487,767 issued to Reid et al. on Dec. 3, 2002. The radial expansion process helps to prevent the blind fastener from rotating or “torquing out” when torque is applied to a screw or bolt mounted in the blind fastener. Thus, the blind fastener remains torsionally static while the accessory bolt or screw is tightened to its requisite design torque or is removed from the blind fastener.
As mentioned, blind fasteners are typically manufactured with a fastener body having an integrally formed closed end. Internal threads are machined within a partial opening in the fastener body. During the machining of the internal threads, metal shavings and other debris can become trapped between the valleys of the threads and accumulate in the region of the closed end. Therefore, the internally threaded region must be cleaned out and at least a majority of the debris removed. One drawback of presently designed blind fasteners is that the integrally formed closed end of the blind fastener prevents the removal of the majority of debris residing in the blind fastener opening after machining. The failure or inability to remove a substantial amount of debris from the threaded region of the blind fastener can cause stress concentration sites in the thread roots, for example, and lead to premature fracturing of the bolt or the blind fastener while the blind fastener is under an operational load. In addition, debris can interfere with the proper threading of an accessory bolt or screw that is inserted into the blind fastener. Debris also causes additional friction between the internal threads of the blind fastener and the threads of the accessory bolt or screw. Increased friction between threaded members can be detrimental to achieving the proper amount of bolt preload. When the preload of a bolt is too low, the accessory device attached via the bolt may vibrate and become loose and/or the bolt would have to carry a larger share of the operational loads, thus reducing the fatigue life of the bolt and possibly leading to premature bolt failure.
One application where blind fasteners can be used is in a so-called “wet” environment, for example, in the structural wall of an airplane fuel tank. Presently, blind fasteners with integrally machined closed ends are used. However, blind fasteners with integrally machined closed ends, referred to as “one-piece blind fasteners,” tend to be costly, tend to be heavy, can be difficult to quality check, and have internal threads that are difficult to clean sufficiently. Another drawback with a one-piece blind fastener is that excessive material is required to allow a sufficient threading depth within the partial opening of the fastener body, this it is often difficult to meet the design requirements for thread runout, etc. Therefore, a low-cost, inspectable, lighter weight, and easier to clean blind fastener that can be used in a wet environment is desirable.